The invention relates to an electroscopic fluid picture-display device provided with display elements arranged in a panel in the form of a matrix of rows and columns, and with a control voltage generator. The display elements each have a first and a second electrode and between them a third electrode that can be moved in a fluid, and the control voltage generator for supplying control voltages is coupled to the electrodes. Depending on the value of the voltage on the movable third electrode relative to that of the first and second electrode, the third electrode is located near the first or to the second electrode. At least one of the three voltages for each display element results in a pulse duration modulation during the display which depends on the value of a video signal to be displayed by television means. To this end the device is provided with a sample-and-hold circuit for the video signal and with a pulse duration modulator following it.
Such a picture-display device is disclosed in Netherlands patent application No. 82 00 354. As a passive display the device operates with ambient light. Use can thereby be made of the reflection or transmission of light by known methods. Light reflection is described whereby light is reflected by the reflecting third electrode when located near to the transparent first electrode, and light is absorbed in an opaque fluid in the display element when the third electrode is close to the second electrode. The first electrodes may be made to serve as a common upper electrode for all display elements, the second electrodes as strip-shaped lower electrodes lying in the row direction, and the third movable electrodes in between providing the electrical interconnection in the column direction. By means of the sample-and-hold circuit and the pulse duration modulator, the third electrode is situated close to the first electrode during a part of a television field period or of a frame period in interlaced television, and is situated during the remaining part of the period close to the second electrode, which parts of the period depend on the local picture signal value. For the pulse duration modulator a clock pulse counter is proposed which, depending on the stored picture signal value, determines the part of the field period in which the third electrode is driven. The result is a television picture with many luminance levels for display.
The pulse duration modulation, depending on the picture signal value, can be a good solution for an electroscopic picture-display device suitable for television with many luminance levels. However, the use of a fluid in the display elements can give rise to problems. For instance, the fluid may be subject to electrolysis when a direct voltage component is present between adjacent electrodes. The products of the electrolysis may lead directly or after some time to an unacceptable change in the properties of the fluid. Another problem is that the direct voltage components between the electrodes may lead to charge accumulation, producing electrical charging and discharging effects in the fluid which may unacceptably affect the position and speed of displacement of the movable third electrode. Since television picture signals contain, in the known way, direct voltage components that depend on the content of the picture signal, pulse duration modulation cannot straight-forwardly be applied in electroscopic picture-display devices that contain display elements filled with a fluid, if the fluid may be affected by direct voltages.